The present invention relates to an oil filter which can be fitted into a reservoir tank and can be connected to the suction line or to the return line of a hydraulic system and to a canister for use in such an oil filter.
An oil filter may be positioned at the point in an oil operated hydraulic system where an oil suction or oil return pipe enters the reservoir tank. The oil may be filtered through a filter canister which is a screw fitting on a filter head or housing mounted on the outer surface of the tank and which projects outwardly therefrom. Examples of such filter canisters primarily for use as oil filters for internal combustion engines are shown in British Pat. Nos. 920,151 (Fram Corporation), 1,017,781 (Puralator Products Inc.) 1,045,288 (Fram Corporation), 1,052,221 (Chevron Research Co.), and 1,429,472 (General Motors Ltd.) Such disposable canisters generally have an end plate formed with oil inlet apertures disposed in a ring and a central threaded oil outlet aperture. A filter head for use with such canisters generally has an annular recess communicating with an oil inlet pipe and an externally threaded centrally positioned oil outlet spigot. In use the threaded central aperture of the canister is screwed onto the oil outlet spigot, the ring of oil inlet apertures registering with the annular recess, and a rubber sealing ring on the end plate being compressed against a region of the filter head surrounding the recess to seal the canister to the filter head. However, the use of an external disposable canister can be undesirable in hydraulic systems on the general grounds of vulnerability and bulk. During the operation of certain types of hydraulic system, distortion of the canister tends to fatigue the end plate to canister joint and may also cause the cover plate and rubber sealing ring to move away from the regions of the filter head against which it is intended to seat which reduces the effectiveness of the oil seal. Furthermore, removal of a spent canister may be made difficult because friction between the rubber sealing ring and the filter head opposes the torque applied to the canister to unscrew it. A strap wrench or other suitable tool may be needed to ease the removal of the canister from the housing.
It has also been proposed to fit within the reservoir tank an internal filter which has a removable filter basket. This arrangement suffers from the disadvantage that the whole filter assembly has to be dismantled each time the filter element needs to be changed, and there is also a risk that contaminated oil may reflux back into the tank during the change of filter element.